This invention relates to the synchronization of variable length encoded video signals.
Variable length encoding, as is well known in the art, provides an efficient means for transmitting digitally encoded signal samples that have unequal probabilities of occurrence. Accordingly, sample levels that have a higher probability of occurrence are assigned code words that have fewer numbers of bits than the words assigned to the less frequently occurring sample levels. The average code word length is determined by a weighting of each word in the alphabet of code words with its expected relative occurrence. The average code word length of a variable length code will thus be less than the code word length required for fixed length encoding. A differentially encoded PCM signal, therefore, in which the expected frequency of signal sample levels is generally unequally distributed, can be efficiently encoded using variable length encoding techniques.
The use of variable length encoding requires that certain constraints be placed on the coding procedures so that each digital word can be unambiguously detected in a bit stream of concatenated code words. Accordingly, each variable length digital word is chosen so that the digits that comprise a shorter length variable length code word cannot occur within a longer code word. Thus, in a bit stream containing a string of variable bit length words, each word can be individually identified and decoded.
A transmission error in a stream of variable length words will, however, cause a decoder to lose word synchronization. When word synchronization is lost while decoding a variable length word digital stream, data outside the code word containing the error may also be improperly decoded. That is, a variable length decoder examines a string of digits until it is determined that a digital string is a word in the alphabet of possible transmitted words. Therefore, if a transmission error causes a digit inversion in the received digital sequence, the variable length decoder could determine that a sequence of digits shorter than the digits in the transmitted word forms a different word. When the decoder thereupon commences to examine the digits that follow, further decoding errors will result until the decoder regains word synchronization. Similarly, as a result of a transmission error, the decoder may continue to examine a string of digits beyond the number of digits in the transmitted word as it tries to detect a word in the alphabet of possible words. Again, until word synchronization is regained, there will be decoding errors in words other than the word in which the transmission error occurred.
Proper word synchronization is critical when, for example, samples are representations of the magnitude of a signal having spatial coordinates, such as a video or facsimile signal. A loss of word synchronization by the decoder will cause a miscount of received data words resulting in improperly addressed code words. A spatially unsynchronized scrambled decoding picture will thereupon result.